Concept 2: Any flight in which you carry customer property (even a single empty container) must be conducted under the provisions of Part 135 of the Federal Aviation Regulations. Any flight in which you carry passengers for hire is also a Part 135 flight. Company materials (“comat”) are not customer property. Carrying Bob's Flying Service employees (such as pilots, mechanics or dispatchers) as passengers does not count as carrying passengers for hire, because they are not paying to fly.
Concept 3: A flight with no customer property and no paying passengers on board may be conducted under Part 91. In that case, Part 135 regulations do not apply to that leg. If a duty assignment is scheduled to include even one Part 135 flight (leg), however, then §135.267 applies to the entire assignment in terms of whether you can accept it or not. In other words, “one drop poisons the whole barrel.” But . . .
Concept 4: If a duty period is not scheduled to include any Part 135 flying, then obviously §135.267 becomes totally irrelevant! That’s right, there are no rules! Under Part 91, there are no restrictions on flight, duty or rest times . . . none whatsoever. The FAA did not write any such regulations. Breaking Part 135 rules when you aren’t operating under Part 135 would be like breaking the laws of Egypt when you’re not in Egypt. Who cares?